The double conversion topology has been the uninterruptible power supply (UPS) standard for high performance topologies for the last 30 years. Double conversion provides the ultimate isolation of the critical load from the anomalies present on the utility electric lines. Some drawbacks of the double conversion topology are that the input rectifiers tend to draw currents at poor power factors and also tend to introduce harmonics, and performance is inconsistent throughout the load range. Filters improve the performance of double conversation systems but do not remedy it.
Recently, another topology has been presented for use as a high performance topology suitable for critical loads. The utility interactive topology does indeed have appeal as it provides the ideal load to the utility, drawing power at near unity power factor and at low harmonic distortion throughout its load range. Additionally, utility interactive topologies exhibit greater efficiencies than the classic double conversion topologies for certain load types. On the downside, utility interactive topologies do not protect the critical load as well from utility line anomalies. Among other shortcomings, certain types of utility faults will pass through the utility interactive system and disturb the critical load. The occasional passing through of utility line faults with this topology is due to the inability of the input coupling switch to disconnect in a quick enough fashion. Ordinary inverse-parallel connected thyristors are often used for this purpose due to their characteristics that make them very well suited for this application. Unfortunately, one drawback of thyristors is that they cannot achieve the off or blocking state without having the applied current achieve a zero value.
An object of the present invention is to ameliorate the system output waveform disruptions that occur due to input utility line disturbances. The pass through of disturbances that can not be sufficiently attenuated by the normal operation of the utility interactive power converter can only be stopped by rapid disconnection of the input coupling switch means. Two techniques are disclosed that accomplish this. The first technique is suitable for use with switches that require the current to achieve zero for commutation, such as thyristors-Silicon Controlled Rectifiers (SCRs), for example. This technique operates one or both of the inverters in such a manner to cause the line current to achieve zero value in a time period during which the net output disturbance is of little concern. The second technique uses a switch that can be self commutated or uses a non self-commutating switch with auxiliary commutation circuit elements. Also disclosed are several variations of a voltage clamping scheme that is required to deal with the energy released by the line circuit inductances during commutation.